Documentation: filesystems: convert vfat.txt to RST

Converts vfat.txt to the reStructuredText format, improving presentation
without changing the underlying content.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
-----------------------------------------------------------
Changes in v3:
Removed unnecessary markup.
Removed section "BUG REPORTS" as recommended by the maintainer.

Changes in v2:
Refactored long lines as pointed out by Jonathan
Copied the maintainer
Updated the reference in the MAINTAINERS file for vfat

I did not move this into admin-guide, waiting on what the
maintainer has to say about this and also about old sections
in the text, if any.

Link: https://lore.kernel.org/r/20191223010030.434902-1-dwlsalmeida@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 387 insertions, 0 deletions

diff --git a/Documentation/filesystems/vfat.rst b/Documentation/filesystems/vfat.rst
new file mode 100644
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+====
+VFAT
+====
+
+USING VFAT
+==========
+
+To use the vfat filesystem, use the filesystem type 'vfat'.  i.e.::
+
+  mount -t vfat /dev/fd0 /mnt
+
+
+No special partition formatter is required,
+'mkdosfs' will work fine if you want to format from within Linux.
+
+VFAT MOUNT OPTIONS
+==================
+
+**uid=###**
+	Set the owner of all files on this filesystem.
+	The default is the uid of current process.
+
+**gid=###**
+	Set the group of all files on this filesystem.
+	The default is the gid of current process.
+
+**umask=###**
+	The permission mask (for files and directories, see *umask(1)*).
+	The default is the umask of current process.
+
+**dmask=###**
+	The permission mask for the directory.
+	The default is the umask of current process.
+
+**fmask=###**
+	The permission mask for files.
+	The default is the umask of current process.
+
+**allow_utime=###**
+	This option controls the permission check of mtime/atime.
+
+		**-20**: If current process is in group of file's group ID,
+                you can change timestamp.
+
+		**-2**: Other users can change timestamp.
+
+	The default is set from dmask option. If the directory is
+	writable, utime(2) is also allowed. i.e. ~dmask & 022.
+
+	Normally utime(2) checks current process is owner of
+	the file, or it has CAP_FOWNER capability. But FAT
+	filesystem doesn't have uid/gid on disk, so normal
+	check is too unflexible. With this option you can
+	relax it.
+
+**codepage=###**
+	Sets the codepage number for converting to shortname
+	characters on FAT filesystem.
+	By default, FAT_DEFAULT_CODEPAGE setting is used.
+
+**iocharset=<name>**
+	Character set to use for converting between the
+	encoding is used for user visible filename and 16 bit
+	Unicode characters. Long filenames are stored on disk
+	in Unicode format, but Unix for the most part doesn't
+	know how to deal with Unicode.
+	By default, FAT_DEFAULT_IOCHARSET setting is used.
+
+	There is also an option of doing UTF-8 translations
+	with the utf8 option.
+
+.. note:: ``iocharset=utf8`` is not recommended. If unsure, you should consider
+	  the utf8 option instead.
+
+**utf8=<bool>**
+	UTF-8 is the filesystem safe version of Unicode that
+	is used by the console. It can be enabled or disabled
+	for the filesystem with this option.
+	If 'uni_xlate' gets set, UTF-8 gets disabled.
+	By default, FAT_DEFAULT_UTF8 setting is used.
+
+**uni_xlate=<bool>**
+	Translate unhandled Unicode characters to special
+	escaped sequences.  This would let you backup and
+	restore filenames that are created with any Unicode
+	characters.  Until Linux supports Unicode for real,
+	this gives you an alternative.  Without this option,
+	a '?' is used when no translation is possible.  The
+	escape character is ':' because it is otherwise
+	illegal on the vfat filesystem.  The escape sequence
+	that gets used is ':' and the four digits of hexadecimal
+	unicode.
+
+**nonumtail=<bool>**
+	When creating 8.3 aliases, normally the alias will
+	end in '~1' or tilde followed by some number.  If this
+	option is set, then if the filename is
+	"longfilename.txt" and "longfile.txt" does not
+	currently exist in the directory, longfile.txt will
+	be the short alias instead of longfi~1.txt.
+
+**usefree**
+	Use the "free clusters" value stored on FSINFO. It will
+	be used to determine number of free clusters without
+	scanning disk. But it's not used by default, because
+	recent Windows don't update it correctly in some
+	case. If you are sure the "free clusters" on FSINFO is
+	correct, by this option you can avoid scanning disk.
+
+**quiet**
+	Stops printing certain warning messages.
+
+**check=s|r|n**
+	Case sensitivity checking setting.
+
+	**s**: strict, case sensitive
+
+	**r**: relaxed, case insensitive
+
+	**n**: normal, default setting, currently case insensitive
+
+**nocase**
+	This was deprecated for vfat. Use ``shortname=win95`` instead.
+
+**shortname=lower|win95|winnt|mixed**
+	Shortname display/create setting.
+
+	**lower**: convert to lowercase for display,
+	emulate the Windows 95 rule for create.
+
+	**win95**: emulate the Windows 95 rule for display/create.
+
+	**winnt**: emulate the Windows NT rule for display/create.
+
+	**mixed**: emulate the Windows NT rule for display,
+	emulate the Windows 95 rule for create.
+
+	Default setting is `mixed`.
+
+**tz=UTC**
+	Interpret timestamps as UTC rather than local time.
+	This option disables the conversion of timestamps
+	between local time (as used by Windows on FAT) and UTC
+	(which Linux uses internally).  This is particularly
+	useful when mounting devices (like digital cameras)
+	that are set to UTC in order to avoid the pitfalls of
+	local time.
+
+**time_offset=minutes**
+	Set offset for conversion of timestamps from local time
+	used by FAT to UTC. I.e. <minutes> minutes will be subtracted
+	from each timestamp to convert it to UTC used internally by
+	Linux. This is useful when time zone set in ``sys_tz`` is
+	not the time zone used by the filesystem. Note that this
+	option still does not provide correct time stamps in all
+	cases in presence of DST - time stamps in a different DST
+	setting will be off by one hour.
+
+**showexec**
+	If set, the execute permission bits of the file will be
+	allowed only if the extension part of the name is .EXE,
+	.COM, or .BAT. Not set by default.
+
+**debug**
+	Can be set, but unused by the current implementation.
+
+**sys_immutable**
+	If set, ATTR_SYS attribute on FAT is handled as
+	IMMUTABLE flag on Linux. Not set by default.
+
+**flush**
+	If set, the filesystem will try to flush to disk more
+	early than normal. Not set by default.
+
+**rodir**
+	FAT has the ATTR_RO (read-only) attribute. On Windows,
+	the ATTR_RO of the directory will just be ignored,
+	and is used only by applications as a flag (e.g. it's set
+	for the customized folder).
+
+	If you want to use ATTR_RO as read-only flag even for
+	the directory, set this option.
+
+**errors=panic|continue|remount-ro**
+	specify FAT behavior on critical errors: panic, continue
+	without doing anything or remount the partition in
+	read-only mode (default behavior).
+
+**discard**
+	If set, issues discard/TRIM commands to the block
+	device when blocks are freed. This is useful for SSD devices
+	and sparse/thinly-provisoned LUNs.
+
+**nfs=stale_rw|nostale_ro**
+	Enable this only if you want to export the FAT filesystem
+	over NFS.
+
+		**stale_rw**: This option maintains an index (cache) of directory
+		*inodes* by *i_logstart* which is used by the nfs-related code to
+		improve look-ups. Full file operations (read/write) over NFS is
+		supported but with cache eviction at NFS server, this could
+		result in ESTALE issues.
+
+		**nostale_ro**: This option bases the *inode* number and filehandle
+		on the on-disk location of a file in the MS-DOS directory entry.
+		This ensures that ESTALE will not be returned after a file is
+		evicted from the inode cache. However, it means that operations
+		such as rename, create and unlink could cause filehandles that
+		previously pointed at one file to point at a different file,
+		potentially causing data corruption. For this reason, this
+		option also mounts the filesystem readonly.
+
+	To maintain backward compatibility, ``'-o nfs'`` is also accepted,
+	defaulting to "stale_rw".
+
+**dos1xfloppy  <bool>: 0,1,yes,no,true,false**
+	If set, use a fallback default BIOS Parameter Block
+	configuration, determined by backing device size. These static
+	parameters match defaults assumed by DOS 1.x for 160 kiB,
+	180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
+
+
+
+LIMITATION
+==========
+
+The fallocated region of file is discarded at umount/evict time
+when using fallocate with FALLOC_FL_KEEP_SIZE.
+So, User should assume that fallocated region can be discarded at
+last close if there is memory pressure resulting in eviction of
+the inode from the memory. As a result, for any dependency on
+the fallocated region, user should make sure to recheck fallocate
+after reopening the file.
+
+TODO
+====
+Need to get rid of the raw scanning stuff.  Instead, always use
+a get next directory entry approach.  The only thing left that uses
+raw scanning is the directory renaming code.
+
+
+POSSIBLE PROBLEMS
+=================
+
+- vfat_valid_longname does not properly checked reserved names.
+- When a volume name is the same as a directory name in the root
+  directory of the filesystem, the directory name sometimes shows
+  up as an empty file.
+- autoconv option does not work correctly.
+
+
+TEST SUITE
+==========
+If you plan to make any modifications to the vfat filesystem, please
+get the test suite that comes with the vfat distribution at
+
+`<http://web.archive.org/web/*/http://bmrc.berkeley.edu/people/chaffee/vfat.html>`_
+
+This tests quite a few parts of the vfat filesystem and additional
+tests for new features or untested features would be appreciated.
+
+NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
+=============================================
+This documentation was provided by Galen C. Hunt gchunt@cs.rochester.edu and
+lightly annotated by Gordon Chaffee.
+
+This document presents a very rough, technical overview of my
+knowledge of the extended FAT file system used in Windows NT 3.5 and
+Windows 95.  I don't guarantee that any of the following is correct,
+but it appears to be so.
+
+The extended FAT file system is almost identical to the FAT
+file system used in DOS versions up to and including *6.223410239847*
+:-).  The significant change has been the addition of long file names.
+These names support up to 255 characters including spaces and lower
+case characters as opposed to the traditional 8.3 short names.
+
+Here is the description of the traditional FAT entry in the current
+Windows 95 filesystem::
+
+        struct directory { // Short 8.3 names
+                unsigned char name[8];          // file name
+                unsigned char ext[3];           // file extension
+                unsigned char attr;             // attribute byte
+		unsigned char lcase;		// Case for base and extension
+		unsigned char ctime_ms;		// Creation time, milliseconds
+		unsigned char ctime[2];		// Creation time
+		unsigned char cdate[2];		// Creation date
+		unsigned char adate[2];		// Last access date
+		unsigned char reserved[2];	// reserved values (ignored)
+                unsigned char time[2];          // time stamp
+                unsigned char date[2];          // date stamp
+                unsigned char start[2];         // starting cluster number
+                unsigned char size[4];          // size of the file
+        };
+
+
+The lcase field specifies if the base and/or the extension of an 8.3
+name should be capitalized.  This field does not seem to be used by
+Windows 95 but it is used by Windows NT.  The case of filenames is not
+completely compatible from Windows NT to Windows 95.  It is not completely
+compatible in the reverse direction, however.  Filenames that fit in
+the 8.3 namespace and are written on Windows NT to be lowercase will
+show up as uppercase on Windows 95.
+
+.. note:: Note that the ``start`` and ``size`` values are actually little
+          endian integer values.  The descriptions of the fields in this
+          structure are public knowledge and can be found elsewhere.
+
+With the extended FAT system, Microsoft has inserted extra
+directory entries for any files with extended names.  (Any name which
+legally fits within the old 8.3 encoding scheme does not have extra
+entries.)  I call these extra entries slots.  Basically, a slot is a
+specially formatted directory entry which holds up to 13 characters of
+a file's extended name.  Think of slots as additional labeling for the
+directory entry of the file to which they correspond.  Microsoft
+prefers to refer to the 8.3 entry for a file as its alias and the
+extended slot directory entries as the file name.
+
+The C structure for a slot directory entry follows::
+
+        struct slot { // Up to 13 characters of a long name
+                unsigned char id;               // sequence number for slot
+                unsigned char name0_4[10];      // first 5 characters in name
+                unsigned char attr;             // attribute byte
+                unsigned char reserved;         // always 0
+                unsigned char alias_checksum;   // checksum for 8.3 alias
+                unsigned char name5_10[12];     // 6 more characters in name
+                unsigned char start[2];         // starting cluster number
+                unsigned char name11_12[4];     // last 2 characters in name
+        };
+
+
+If the layout of the slots looks a little odd, it's only
+because of Microsoft's efforts to maintain compatibility with old
+software.  The slots must be disguised to prevent old software from
+panicking.  To this end, a number of measures are taken:
+
+        1) The attribute byte for a slot directory entry is always set
+           to 0x0f.  This corresponds to an old directory entry with
+           attributes of "hidden", "system", "read-only", and "volume
+           label".  Most old software will ignore any directory
+           entries with the "volume label" bit set.  Real volume label
+           entries don't have the other three bits set.
+
+        2) The starting cluster is always set to 0, an impossible
+           value for a DOS file.
+
+Because the extended FAT system is backward compatible, it is
+possible for old software to modify directory entries.  Measures must
+be taken to ensure the validity of slots.  An extended FAT system can
+verify that a slot does in fact belong to an 8.3 directory entry by
+the following:
+
+        1) Positioning.  Slots for a file always immediately proceed
+           their corresponding 8.3 directory entry.  In addition, each
+           slot has an id which marks its order in the extended file
+           name.  Here is a very abbreviated view of an 8.3 directory
+           entry and its corresponding long name slots for the file
+           "My Big File.Extension which is long"::
+
+                <proceeding files...>
+                <slot #3, id = 0x43, characters = "h is long">
+                <slot #2, id = 0x02, characters = "xtension whic">
+                <slot #1, id = 0x01, characters = "My Big File.E">
+                <directory entry, name = "MYBIGFIL.EXT">
+
+
+           .. note:: Note that the slots are stored from last to first.  Slots
+		     are numbered from 1 to N.  The Nth slot is ``or'ed`` with
+		     0x40 to mark it as the last one.
+
+        2) Checksum.  Each slot has an alias_checksum value.  The
+           checksum is calculated from the 8.3 name using the
+           following algorithm::
+
+                for (sum = i = 0; i < 11; i++) {
+                        sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
+                }
+
+
+	3) If there is free space in the final slot, a Unicode ``NULL (0x0000)``
+	   is stored after the final character.  After that, all unused
+	   characters in the final slot are set to Unicode 0xFFFF.
+
+Finally, note that the extended name is stored in Unicode.  Each Unicode
+character takes either two or four bytes, UTF-16LE encoded.